Insufficiently inflated or over-inflated tires for vehicles present an often unknown danger to drivers and passengers of the vehicles. This improper inflation can cause poor handling, poor traction, reduced fuel efficiency and can cause tire failures if left improperly inflated for a period of time. A visual inspection of a vehicle's tires can provide some indication of the level of inflation of the tires. However, this method is extremely inaccurate and newer “run flat” tires often retain some degree of structural integrity even after losing inflation pressure making a visual inspection of the internal tire pressure nearly impossible. Further, it is suspected that many vehicle owners often neglect to examine the level of inflation in their vehicle's tires for extended periods of time. Therefore, it is desirable to provide a vehicle with an internal tire pressure sensing system for determining the level of inflation in the vehicle's tires and alerting the operator if the level is outside a preselected operating range.
Several solutions to the problem of detecting and monitoring tire pressure have been proposed in the past. Typically, known tire pressure sensors include a transmitter mounted within the tire (often to the wheel) adjacent to, or integral with, a valve stem therefor. Because the tire pressure sensors detect and transmit a value proportional to the pressure in the tire, this value is typically normalized to compensate for variations in the ambient temperature to prevent inaccurate readings due to air expansion from temperature variations. Further, tires tend to heat up after extended periods of use, also requiring correction for temperature variations. Some examples of tire pressure sensors which include correction for temperature variations are taught by U.S. Pat. Nos. 4,567,459, 4,703,650, and 4,966,034.
Due to the physical constraints presented by the location of the tire pressure sensor within the tire and the impracticability of running wire leads from the tire pressure sensor to a control unit within the vehicle, it is desirable to employ a wireless transmission system to relay the data output by the tire pressure sensor to the control unit. In addition to the above-mentioned patents, examples of wireless transmission systems employing means such as radio transmitters are shown in U.S. Pat. Nos. 4,510,484, 4,554,527, and 5,061,917. The tire pressure sensors can include a wireless transmitter-such as that shown in U.S. Pat. No. 4,978,941. In addition to radio transmissions, each of the vehicle's tires can be coded with a unique digital value such as that shown in U.S. Pat. Nos. 5,001,457 and 5,061,917.
Referring now to the drawings and to FIG. 1 in particular, a vehicle 10 is shown having a generally well-known configuration: four ground-engaging wheels 12 with a spare tire 14 located in a trunk portion 16 of the vehicle 10. The vehicle 10 is also provided with conventional windshields, one forward windshield 18 and one rearward windshield 20. The forward windshield 18 is provided with a conventional rear view mirror 22. A driver-side external rear view mirror 24 and a passenger-side rear view mirror are located adjacent the forward windshield 18, typically positioned on corresponding front doors (not shown) of the vehicle 10. Each of the external mirrors 24 and 26 is preferably provided with an antenna system. An example of a prior art antenna system in which a radio frequency antenna is mounted within an exterior mirror for a vehicle is shown in commonly-assigned U.S. Pat. No. 5,504,478 to Knapp, issued Apr. 2, 1996 and is incorporated herein by reference.
Several problems have been encountered with the known wireless tire pressure sensor systems. These systems require internal calibration to ensure proper display of information and typically need recalibration if transmitters (or the tires they are mounted to) are replaced or rotated. This calibration and recalibration is an inevitable consequence of the requirement for a uniquely coded transmitter corresponding to the location of the tire on the vehicle. Thus, the system knows if it is detecting a front drivers-side tire pressure, a rear passengers-side tire pressure, etc., depending upon the code detected by the system.
U.S. Pat. No. 5,600,301 shows an example of a remote tire pressure sensing system wherein each of the tire pressure sensors has a transmitter provided with a unique code at manufacture.
FIG. 2 is a perspective view of an example of a wheel 12 for the vehicle 10 provided with a prior art tire pressure sensor 28 adapted to send a signal detectable by an antenna system on one of the external rear view mirrors 24 and 26. Each of the wheels 12 (and the spare tire 14) of the vehicle 10 are preferably provided with a tire pressure sensor 28 which is adapted to transmit a signal corresponding to the pressure within the wheel 12 (or the spare tire 14).
The tire pressure sensor 28 is shown in greater detail in FIG. 3 and preferably comprises a body 30 having a valve stem 32. The valve stem 32 is used to inflate or exhaust pressurized air from within the wheel 12 (when encased by a conventional tire). The body 30 preferably contains a well-known pressure sensor and circuitry adapted to transmit a signal corresponding to the pressure detected by the tire pressure sensor 28. Each tire pressure sensor 28 thereby must transmit the unique code in addition to the pressure signal to a receiver at different intervals to provide an indication of the tire pressure in each of the tires.
One problem with the above-described system is that an initialization procedure must be performed to determine the location of each particular tire pressure sensor on the vehicle. Each transmitter is designed to transmit tire pressure and identification data when a magnet is held in close proximity. The initialization process is performed by placing the receiver in a learning mode and then triggering the transmitter in each tire pressure sensor with a magnet in a predetermined sequence. In this manner, the receiver can associate each tire pressure sensor's unique code during the initialization sequence to determine the tire's relative position on the vehicle. As an example, the initialization is begun by triggering the front driver-side tire transmitter first and then triggering each of the other tire transmitters in a counterclockwise sequence around the vehicle.
Several additional problems have been encountered with this prior art system. First, each transmitter requires a magnetic sensor to “activate” it during the initialization process adding cost to the transmitter and requiring additional space within the transmitter.
Second, the system must be recalibrated when tires are rotated or replaced causing inconvenience to the vehicle operator and necessitating at least one performance of the initialization process to enable the receiver to “relearn” the location of the tire pressure sensors.
Third, the receiving antenna is likely required to be omni-directional and centrally located within the vehicle, requiring increased signal strength from each transmitter due to the signal shielding effect from the vehicle's structure, including radio frequency shielding windows such as “tinted”, low e value windows, requiring higher transmitter power and/or a higher receiver sensitivity. A higher power transmitter can reduce battery life and add to the system cost. A higher receiver sensitivity can increase cost and increase susceptibility to unwanted signals.
Fourth, because the transmitters are not synchronized, it is possible that two or more transmissions could occur at nearly the same instant, making the transmitted signals undecipherable, especially with a centrally-located antenna.
Fifth, the transmitter in each tire pressure sensor in the system described above requires a unique code so that the receiver can distinguish each transmitter's relative position after initialization. As the number of vehicles equipped with these types of tire pressure sensing systems increases, a data frame of increased length (typically additional digits or characters) would be required to ensure uniqueness, thereby decreasing battery life due to increased use.
Sixth, with systems having an omni-directional antenna, the instant vehicle and vehicles adjacent to it would result in cross-detection of tire pressure sensor transmissions between adjacent vehicles resulting in erroneous pressure data.
Further, wireless transmission of other vehicle diagnostic data is becoming more common which increases the probability that the various signals transmitted around the vehicle's interior and exterior will cause interference and thus reduce the effectiveness of the various wireless transmission systems. Also, while attention has been paid in the past to selection of appropriate transmission frequencies to avoid interference from other radio frequency sources, particular problems are presented by remote control systems, such as keyless entry systems, garage door openers, etc., which are becoming more and more widely implemented. These exterior transmission signals can also cause additional interference and require additional antenna arrays to be built into the vehicle receiving systems.